. >> reporter: the fbi is leading the investigation. their team now on the ground in libya responsible for collecting the evidence intended to help whittle down that suspect list. >> we are conducting interviews, gathering evidence and trying to sort out the facts working with our partners both from a criminal standpoint as well as in the intelligence community to try to determine exactly what took place on the ground that evening. >> reporter: there are significant challenges facing u.s. investigators and the intelligence community. for one, getting a level of granularity that will allow them to identify individuals and their associations with various groups. another challenge, sifting through whatever information or evidence was left behind at a crime scene that was never really secured. all against a backdrop of concern for the investigators' safety. >> the fbi has a track record of being able to go into these places that are volatile and be able to put together a criminal case. we've done it in yemen with the coal bombing. we did it